Synthetic, commercially available brown pigments are largely obtained by mixing yellow, red and/or black iron oxide pigments (Ullmanns Encyklopadie der technischen Chemie, 4th revised and extended Edition, Vol. 18, Anorganische Pigmente (Inorganic Pigments), 1979, page 603). The presence of iron oxide yellow and/or black makes the products thermally unstable, which limits their range of application. The presence of hematite in the brown pigments in their lightened form, for example in admixture with TiO.sub.2, results in an unwanted, violet hue, which further restricts their range of applications.
The incorporation of manganese in the .alpha.-Fe.sub.2 O.sub.3 lattice (U.S. Pat. No. 3,276,894 and DE-A 881 562) gives temperature-stable brown pigments, although the violet hue remains intact in the lightened form.
Although the production of iron/aluminium mixed oxides in accordance with DE-A 3 324 400 leads to brown pigments having good properties, the mixed precipitates obtained are difficult to filter, which complicates the production process. In addition, the high calcination temperature makes the products grain-hard so that they have to be intensively ground.
.gamma.-Fe.sub.2 O.sub.3 has long been known as a magnetic pigment (cf. U.S. Pat. No. 3,082,067). JP-A 61-232 223 describes isometric brown pigments based on .gamma.-Fe.sub.2 O.sub.3 with a content of SiO.sub.2. In admixture with TiO.sub.2, however, these brown pigments show an undesirable red tinge.
Although .gamma.-FeOOH has been described as pure brown (U.S. Pat. Nos. 3,382,174 and 2,560,970), commercial .gamma.-FeOOH pigments show a yellow-orange hue. These products decompose to red-brown .gamma.-Fe.sub.2 O.sub.3 at temperatures above 200.degree. C. (U.S. Pat. No. 3,082,067) and to red .alpha.-Fe.sub.2 O.sub.3 at temperatures above 300.degree. C. (U.S. Pat. No. 3,382,174).